The present invention relates to gas turbines in general and particular to turbine blades or buckets having cooling passages within the blades for efficient heat exchange with, and cooling of, the blades.
It is customary in turbines to provide internal cooling passages in the blades or buckets of turbine rotors and it has been recognized that the various stages of the turbine rotors require more or less cooling, depending upon the specific location of the stage in the turbine. The first stage turbine buckets usually require, among the various rotor stages, the highest degree of cooling because those turbine blades are the blades exposed immediately to the hot gases of combustion flowing from the combustors. It has also been recognized that the temperature profile across each turbine blade peaks along an intermediate portion of the blade, i.e., in a stagnation or pitch area., and that the temperatures adjacent the root and tip portions of the blades are somewhat lower than the temperatures along the intermediate portion.
Typically, a plurality of cooling passages are provided within the turbine blades extending from the blade root portion to the tip portion. Cooling air from one of the stages of the compressor is conventionally supplied to these passages to cool the blades. Certain turbine blade designs employ turbulence promoters throughout the entire length of these passages to enhance the heat transfer mechanism between the metal of the blades and the flow of cooling air through these passages. This enhancement of the heat transfer coefficient between the blade material and the cooling air occurs by breaking down the boundary layer of air flowing along the internal passages and hence reducing the resistance to heat transfer caused by the thickness of the boundary layer. Consequently, the turbulence promoters separate the flow of cooling air from the internal wall of the blade, rendering it turbulent and hence mix the cool incoming air with the air near the wall to improve the heat transfer relation. In short, the laminar flow normally associated with smooth bore passages in the turbine blade is converted to a turbulent flow to enhance heat transfer.
A problem with the use of turbulence promoters, however, is that the enhancement in heat transfer is accompanied by an increase in the flow resistance and hence an increase in frictional pressure drop in the cooling passage. The increase in pressure drop, of course, means a conversion of the energy into frictional losses which, in turn, decrease the efficiency of the machine. With turbulence promoters extending the full length of the cooling passages, the pressure drop is increased, resulting in friction losses and cooling in regions along the blade where cooling is not necessary or cooling to the extent provided in sections containing turbulence promoters is not required. Because the local cooling requirements along the length of the turbine blades from the root to the tip portions depend on the local external gas temperatures and heat transfer coefficients, the use of turbulence promoters along the entire length of the cooling passages for the blade generates a heat transfer enhancement in needed, as well as unneeded, portions of the turbine. This creates unnecessary and large pressure losses.
Moreover, the formation of turbulence promoters in the internal cooling passages of a turbine blade is a costly, time-consuming operation. One method employed to form the passages in a turbine blade is known as electrochemical machining (ECM). In that method, the turbine blade is first cast and then drilled from tip to root, using an elongated, thin electrode having a central, passage for flowing a chemical electrolyte. Upon energization of the electrode and application of the electrode tip to the blade tip, the electrode removes the metal to penetrate the tip and form the passage. By changing the residence time in the passage, it is possible to remove additional or lesser quantities of metal, as necessary.
According to the present invention, the cooling passages of a turbine blade are provided with turbulence promoters at preferential areas along the length of the airfoil from the root to the tip portions, depending upon the local cooling requirements along the blade. Because the temperature profile of a turbine blade is such that an intermediate region between the root and tip portions is the hottest portion of the blade (the root and tip portions being somewhat cooler), the turbulence promoters are preferentially located in this intermediate region of the turbine blade, while the passages through the root and tip portions of the blade remain essentially smooth-bore. It has been found according to the present invention that the increased turbulence in the hottest portion of the blade increases the heat transfer coefficient sufficiently to maintain the material of the blade in that region below its melting temperature. Also, it has been found that the flowing of cooling fluid, e.g., air, in the root and tip portions of the blade is sufficient to cool the blade in those areas to the required temperature without incurring the penalty of an additional pressure drop caused by promoting turbulence in those areas. Consequently, the length of the intermediate portion of the blade and the geometry of the turbulated section is selected in accordance with local cooling requirements along the blade length necessary to maintain the metal wall temperatures within design limits.
In a preferred embodiment according to the present invention, there is provided a blade for a turbine comprising a blade body having a cross-section generally airfoil in shape, with root and tip portions adjacent opposite ends and a portion intermediate the root and tip portions. A plurality of cooling passages extend within the blade body through the root and tip portions and the intermediate portion for conducting cooling fluid along the blade body in heat transfer relation therewith, at least one of the cooling passages having a series of turbulence promoters formed along the intermediate portion to provide a turbulent flow of cooling fluid through the intermediate portion and enhanced heat transfer between the blade body and the cooling fluid flowing through the one passage. The portions of one passage pass; through the root and tip portions having smooth bores to provide substantially non-turbulent flow of cooling fluid through the root and tip portions of one passage.
In a further preferred embodiment according to the present invention, there is provided a rotor blade for a turbine comprising a blade body having a cross-section generally airfoil in shape, with root and tip portions adjacent opposite ends and a portion intermediate the root and tip portions. A plurality of cooling passages extend within the blade body through the root and tip portions and the intermediate portion for conducting cooling fluid along the blade body in heat transfer relation therewith, at least one of the cooling passages having a series of turbulence promoters formed along the intermediate portion to provide a turbulent flow of fluid through the intermediate portion and enhanced heat transfer between the blade body and the cooling fluid flowing through one passage. The turbulence promoters are formed solely along the intermediate section commencing at about 20% of the length of the blade from the root end of the blade and terminating at about 20% of the length of the blade from the tip end of the blade.
In a further preferred embodiment according to the present invention, there is provided a method of forming cooling passages in a turbine blade by an electrochemical machining process having an elongated electrode for penetrating the metal of the blade, comprising the steps of (a) applying the electrode to one end of the blade to penetrate the blade end to form a first cooling passage having a relatively smooth bore, (b) subsequently successively slowing and increasing the rate of penetration of the electrode into the blade whereby the residence time of the tip of the electrode in the blade is successively altered to form successively larger and smaller diameter bore portions at successive locations along the length of the blade and (c) subsequent to step (b), advancing the electrode at a substantially constant rate of penetration to provide a relatively smooth bore portion of cooling passage adjacent the opposite end of the turbine blade.
It is a primary object of the present invention to provide a turbine blade having preferentially located turbulence promoters for enhancing the heat transfer in regions of the blades subjected to the higher temperatures in use whereby pressure losses due to cooling requirements are reduced and efficiency increased. It is a further object of the present invention to provide an improved method of forming cooling passages in turbine blades.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.